The object of the present invention is a loop filter situated in the feeback channel of a frequency synthesizer and consisting of an integrator with a zero in the frequency response in order to achieve stability, and of a low pass filter. Such a frequency synthesizer comprising a loop filter known from FIG. 1 is used for example in radio telephones as a local oscillator. In the synthesizer, frequency is formed in a known way according to FIG. 2 in a voltage-controlled oscillator, the frequency of which has been locked to the frequency of a crystal oscillator used as a comparison frequency. The frequencies of the voltage-controlled oscillator and the crystal oscillator are conveyed via dividers to a phase comparator, from the output of which the signal based on the phase or frequency difference is obtained. Generally, at least one of the dividers is then programmable. From the output of the phase comparator, the signal goes to the loop filter, the control voltage of the oscillator being obtainable from the filter.
In a known loop filter a problem is caused by the filtering of sidebands brought about by the comparison frequency while striving for as fast a loop as possible. The reason for the problems is that the poles (on the frequency level) of the low pass filter near the natural frequency of the loop reduce the phase margin, decreasing thus the damping factor.
Attempts have been made to solve the problems by using a low enough natural loop frequency, or by using a separate acceleration coupling. However, in that case, the dimensioning of the components has to be made so precise that even a small tolerance variation in the components causes the setting time to grow too long, or that the sidebands rise too high. A further drawback is the complex structure of the acceleration coupling.
In these known cases, the loop filter has been formed by using an integrator, as can be seen in connection with the operational amplifier V of FIG. 1. In the feedback channel, resistor R1 and condensators C1 and C2 together with input resistor Ri form an integrating factor. Further, an RC-filter is often coupled after the integrator.